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Comprehensive Description

Mnemiopsis leidyi is a ctenophore that is native to the western Atlantic, but by the late 1980s was established as an invasive exotic in the Black Sea, presumably after crossing the Atlantic in ship ballast water (it has subsequently appeared in the Caspian, Aegean, Azov, Marmara, North, Baltic, Skagerrak, and Mediterranean Seas). It reached very large numbers and depleted stocks of zooplankton as well as fish eggs and larvae, triggering the crash of several fisheries. In 1997, however, another ctenophore native to the western Atlantic, Beroe ovata, was discovered in the northeastern Black Sea. Beroe ovata is known to feed on planktivorous ctenophores and, in particular, on M. leidyi. The arrival of B. ovata appears to have stabilized the Black Sea ecosystem, leading to a reduction in M. leidyi populations and subsequent recovery of plankton and fish populations. (Shiganova et al. 2003 and references therein)

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In its native range, Mnemiopsis leidyi is found from Cape Cod Bay (Massachusetts, U.S.A.) southward and is the most common ctenophore south of Cape Cod. It enters the nearly freshwater parts of estuaries such as Chesapeake Bay. (Gosner 1978)

Mnemiopsis leidyi is native to the Atlantic coast of the United States, but over the past several decades it has invaded the Black, Caspian, Aegean, Azov, Marmara, North, Baltic, and Skagerrak Seas and has recently been reported to be established in the Mediterranean Sea (Faasse and Bayha 2006; Javidpour et al. 2006; Boersma et al. 2007; Reitzel et al. 2007 and references therein; Fuentes et al. 2010; Reusch et al. 2010 and references therein).

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Alien species

The sea walnut Mnemiopsis leidyi is an infamous predator of zooplankton and fish eggs. The species’ natural area of distribution is along the Atlantic coast of North- and South-America. In the 1980's the sea walnut was accidently introduced in the Black Sea as a stowaway in ballast water of cargo ships. The introduction led to a collapse of the ecosystem. Since a couple of years, the sea walnut is also present in the North Sea, and we now have to wait and see what ecological consequences it will bring to the region.

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Alien species

The sea walnut Mnemiopsis leidyi is an infamous predator of zooplankton and fish eggs. The species’ natural area of distribution is along the Atlantic coast of North- and South-America. In the 1980's the sea walnut was accidently introduced in the Black Sea as a stowaway in ballast water of cargo ships. The introduction led to a collapse of the ecosystem. Since a couple of years, the sea walnut is also present in the North Sea, and we now have to wait and see what ecological consequences it will bring to the region.

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Associations

Young of the burrowing anemone Edwardsiella lineata (formerly known as Edwardsia leidyi or Fagesia lineata as a consequence of taxonomic confusion, Daly 2002), which resemble pinkish tentacled worms, are parasitic in the guts of ctenophores, including Mnemiopsis leidyi (Gosner 1978). The ecological and developmental relationships among E. lineata, M. leidyi, and another ctenophore, Beroe ovata, were studied by Reitzel et al. (2007). They found that although E. lineata infects both of these ctenophores, E. lineata larvae proved far more successful at infecting M. leidyi than B. ovata. Furthermore, E. lineata parasites excised from M. leidyi exhibited greater developmental competence than did E. lineata excised from B. ovata. The authors found that, although E. lineata is efficiently transferred from M. leidyi to B. ovata when the latter preys upon the former, E. lineata larvae are not well adapted for parasitizing or feeding in the latter species. Their results strongly suggest that M. leidyi is the preferred host--and possibly the only suitable natural host--for E. lineata. Although in the wild B. ovata can become more heavily infested than M. leidyi with E. lineata, B. ovata nevertheless appears to be an inadvertent host that acquires E. lineata parasites principally, if not exclusively, from feeding on infected M. leidyi. Furthermore, E. lineata’s competence to complete development from the parasite to the adult polyp is affected by both its size and the terminal host it occupies. Development proceeds more quickly and successfully when M. leidyi is the terminal host. (Conveniently for researchers, when E. lineata is excised from its host, it undergoes a rapid developmental transformation, during which it morphs from the nonciliated, vermiform [worm-like] body plan it exhibits as a parasite into the ciliated, fusiform body plan typical of a planula larva. Remarkably, if provided with a second host, the planula can reinfect another ctenophore and revert to the parasite body plan, whereas if it is denied a second host, the planula can develop into a free-living polyp.) (Reitzel et al. 2007 and references therein)

Beroe ovata is a selective predator favoring M. leidyi in locations where the native ranges of these animals overlap, and along with B. ovata it has therefore been suggested as a biological control agent for invasive M. leidyi in the Black Sea (B. ovata has now established itself in much of the non-native range of M. leidyi, rendering planning for its possible use as a control agent largely academic). However, the combined effects of B. ovata and E. lineata on M. leidyi populations are difficult to predict. These effects could be strictly additive, or the 2 species might even act synergistically to drive M. leidyi populations more sharply downward. Either of these interactions could achieve the desired result of controlling M. leidyi. However, if E. lineata has a negative impact on B. ovata populations, particularly if E. lineata impacts B. ovata more negatively than it impacts M. leidyi, then the presence of E. lineata could undermine efforts to control M. leidyi using B. ovata. On the other hand, in the event that E. lineata has a similarly detrimental effect on both M. leidyi and B. ovata, the simultaneous deployment of E. lineata and B. ovata could serve as an effective control on M. leidyi populations that would be self-limiting, as B. ovata blooms could be controlled by the parasitic anemones. This last possibilty seems particularly important given that B. ovata may generalize its ecological niche to include feeding on other gelatinous zooplankton, including native ctenophores and jellyfish. Given the current level of understanding of interactions among these species, Reitzel et al. question the wisdom of any active effort to use E. lineata as a biological control agent against M. leidyi. (Reitzel et al. 2007)

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General Ecology

Ecology

Kremer (1994) reviewed the ecology of Mnemiopsis leidyi and M. mccradyi in Atlantic and Gulf of Mexico coastal waters from Cape Cod to Texas.

In the invaded areas of the Black, Azov and Caspian Seas, large populations of Mnemiopsis leidyi have contributed to major ecological regime shifts from a pelagic system dominated by planktivorous fish to one dominated by gelatinous plankton, including a total collapse of the pelagic fisheries in the 1990s (Shiganova and Bulgakova 2000; Oguz et al. 2008).

In the early 1980s, M. leidyi was introduced to the Black Sea. By 1988, it had spread across the entire Sea and underwent a population explosion in the fall of 1989, with populations fluctuating dramatically in subsequent years. Huge M. leidyi populations decreased the biomass, density, and species diversity of edible zooplankton as well as fish eggs and larvae, the main food of M. leidyi. This, in turn, caused declines in stocks of planktivorous fish (such as anchovy [Engraulis encrasicolus ponticus], horse mackerel [Trachurus mediterraneus ponticus], and, to a lesser extent, sprat [Sprattus sprattus phalericus]). Declines in these fish populations led to declines in piscivorous fish and dolphins feeding mostly on anchovy and sprat. Mnemiopsis leidyi expanded from the Black Sea to the Seas of Azov and Marmara and were regularly carried out to the Aegean Sea with the Black Sea currents. In 1999, M. leidyi was introduced into the Caspian Sea, apparently, with ballast waters of oil tankers. An important factor permitting the explosion of M. leidyi populations was the lack of a predator in its new range. In 1997, however, another ctenophore native to the western Atlantic, Beroe ovata, was discovered in the northeastern Black Sea. Beroe ovata is known to feed on planktivorous ctenophores and, in particular, on M. leidyi. The arrival of Beroe ovata appears to have stabilized the Black Sea ecosystem, leading to a reduction in M. leidyi populations and subsequent recovery of plankton and fish populations. (Shiganova et al. 2003 and references therein)

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Life Cycle

Time Table of Development

At the time of hatching, the embryo is well formed. Four double rows of ciliated plates and two long lateral tentacles are present. The apical sense organ is prominent on the aboral pole. The endodermal gut is connected to a large stomodeum, and has six lateral diverticula: two to the tentacles, and one to each of the four rows of comb-plates. See the paper of Mayer (1912) for diagrams of the larvae.

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Egg Characteristics

Since the animals are hermaphroditic, the eggs are fertilized before shedding occurs. Cleavage is total but unequal; the first two cleavages are longitudinal, cutting from the animal to the vegetal pole. The next cleavage is oblique, producing four large central cells and four small lateral cells which migrate towards the animal pole to form a flat plate of eight cells. These eight large "macromeres" produce an octet of smaller micromeres by a fourth cleavage. A continued production and division of the micromeres leads to an epibolic type of gastrulation.

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Time Table of Development

Since Mnemiopsis is not common at Woods Hole, Mass., until August, it is often difficult to obtain animals in the breeding condition. The adults can best be collected at night with a hand net, although they have been found at the surface of the water on calm days, between the hours of 9 and 11 A.M., and 4 and 6 P.M.

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Time Table of Development

At the time of hatching, the embryo is well formed. Four double rows of ciliated plates and two long lateral tentacles are present. The apical sense organ is prominent on the aboral pole. The endodermal gut is connected to a large stomodeum, and has six lateral diverticula: two to the tentacles, and one to each of the four rows of comb-plates. See the paper of Mayer (1912) for diagrams of the larvae.

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Egg Characteristics

Since the animals are hermaphroditic, the eggs are fertilized before shedding occurs. Cleavage is total but unequal; the first two cleavages are longitudinal, cutting from the animal to the vegetal pole. The next cleavage is oblique, producing four large central cells and four small lateral cells which migrate towards the animal pole to form a flat plate of eight cells. These eight large "macromeres" produce an octet of smaller micromeres by a fourth cleavage. A continued production and division of the micromeres leads to an epibolic type of gastrulation.

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Time Table of Development

Since Mnemiopsis is not common at Woods Hole, Mass., until August, it is often difficult to obtain animals in the breeding condition. The adults can best be collected at night with a hand net, although they have been found at the surface of the water on calm days, between the hours of 9 and 11 A.M., and 4 and 6 P.M.

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Growth

Development

Most adult tentacle-bearing ctenophores (tentaculates) have an excellent ability to regenerate missing body regions (Coonfield 1937 and references therein; Henry and Martindale 2000 and references therein) and are capable of replacing all identified cell types and structures in their correct location regardless of which portions are removed or damaged. Henry and Martindale studied a phenomenon known as "post-regeneration" in Mnemiopsis leidyi and discussed the significance of their findings in terms of the organization of the ctenophore body plan and the mechanisms involved in cell fate specification. In post-regeneration, deficient embryos generate incomplete larval or adult body plans in which no embryonic regulation (self-correction) has apparently taken place. Subsequently, regeneration of the missing structures occurs in the larva or adult, which is somehow able to “detect” the missing structures, even though these were never present to begin with. Because no injury is required to initiate the post-regenerative effort (as would be necessary, by definition, for ordinary regeneration), the phenomenon of post-regeneration suggests that some intrinsic map of the complete body plan exists within these partial animals. The authors note that the beroids (atentaculates), which do not form tentacles during their development, are not capable of post-regeneration. They speculate about possible developmental mechanisms that might explain this difference between the tentaculate and atentaculate ctenophores. (Henry and Martindale 2000)

Henry and Martindale (2001) report on a study using cell lineage and cell deletion techniques to investigate cell interactions in key aspects of Mnemiopsis leidyi development.

Pang and Martindale (2008) isolated seven homeobox genes from M. leidyi and examined their expression through development. They found that most of these homeobox genes begin expression at gastrulation and that their expression patterns suggest a possible role in patterning of the tentacle apparati and pharynx.

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Evolution and Systematics

Evolution

Phylogeography

Reusch et al. (2001) used microsatellites to infer the geographic origins of invasive Mnemiopsis leidyi in Eurasia. They concluded that the Mnemiopsis invading the Black and Caspian Seas in the 1980s and 1990s originated from within or close to the Gulf of Mexico, whereas the 2006 invasion of the North and Baltic Seas could be traced directly to the New England region of the United States. Genetic diversity in the Baltic Sea was similar to that in New England, but diversity in the North Sea was reduced, supporting the hypothesis that this ctenophore intitially invaded northern Europe via a Baltic port. There has been some suggestion that Mnemiopsis south of Cape Hatteras, North Carolina (U.S.A.) are M. mccradyi rather than M. leidyi, which would mean that the Mnemiopsis that invaded the Black and Caspian Seas were actually M. mccradyi, but so far most evidence seems to indicate that this is not the case and that in fact all the Eurasian Mnemiopsis invasions have involved M. leidyi (Reusch et al. 2010 and references therein). More generally, Gorokhova and Lehtiniemi (2010) suggested that the identification of ctenophores in the Baltic Sea as Mnemiopsis has not been approached with sufficient rigor, a charge vigorously denied by Javidpour et al. (2010). Faasse and Bayha (2006) also emphasize the care that must be taken in identifying ctenophores, ideally using both morphological and molecular analyses, and suggest that M. leidyi may have been present in Dutch waters for several years prior to their report of its presence, having previously been misidentified as the morphologically similar Bolinopsis infundibulum.

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Risks

Risk Statement

According to McNamara et al. (2010), in its native range along the mid-Atlantic coast of the US, Mnemiopsis leidyi appears to be increasing in abundance and undergoing shifts in its historical seasonal distribution. Recent increases in ctenophore abundance in a variety of areas have been attributed to various marine ecosystem alterations, including localized warming of seawater masses (but apparently not Long Island estuaries, which were the focus of studies by McNamara et al.) and the removal of ctenophore predators (such as butterfish) and competitors (such as zooplanktivorous fishes) by overfishing (McNamara et al. 2010 and references therein). McNamara et al. studied shifting M. leidyi abundance in Long Island (New York, U.S.A.) estuaries and its implications for top-down control of the plankton community. They estimated that at its highest densities M. leidyi can remove an overall average of 20 to 89% per day of bivalve mollusk veliger larvae and other zooplankton taxa, including adult copepods, nauplii (early larvae of certain crustaceans), and tintinnids (a group of ciliate protozoans). The authors suggest that increasing ctenophore abundance, especially during a period when they were not historically abundant (i.e., June) may have significant consequences for species which spawn at this time. For example, current populations of M. leidyi represent a major source of larval mortality for bivalves and may inhibit efforts to recover viable populations of commercially important shellfish such as the hard clam Mercenaria mercenaria in Long Island estuaries.

Contents

Description and ecology

Mnemiopsis have a lobed body that is oval-shaped and transparent, with four rows of ciliated combs that run along the body vertically and glow blue-green when disturbed. They have several feeding tentacles. Unlike cnidarians, Mnemiopsis doesn't sting. Their body contains 97% water. They are small animals, having a maximum body length of roughly 7–12 centimetres (3–5 in) and a diameter of 2.5 centimetres (1 in).

It is euryoecious, tolerating a wide range of salinity (2 to 38 psu), temperature (2–32 °C or 36–90 °F), and water quality.

This comb jelly has the capacity for self fertilization, as they are hermaphroditic. They have gonads that contain the ovary and spermatophore bunches in their gastrodermis. This animal carries 150 eggs along each meridional canal. Eggs and sperm are released into the water column where external fertilization takes place. The spawning commences at late evening or at 1:00 or 2:00 a.m. The spawning eggs develop a thick outer layer within 1 minute after touching the seawater. As many as 10,000 eggs can be produced from large specimens in areas with good prey abundance. Egg production can start when the animals reach about 15 mm in length. Egg production increases with ctenophore size, and it is unclear when senescence occurs.

As an invasive species

1980s – Black Sea

Mnemiopsis leidyi was introduced in the Black Sea in the 1980s, where only one species of comb jelly, the small sea gooseberry Pleurobrachia pileus occurred until then. The most likely cause of its introduction is accidentally by merchant ships' ballast water. The first Black Sea record was in 1982.[3]

By 1989, the Black Sea population had reached the highest level, with some 400 specimens per m³ of water (>10 animals/cubic foot) in optimal conditions.[4] Afterwards, due to depletion of foodstocks resulting in lower carrying capacity, the population dropped somewhat.

In 1999 the species was introduced in the Caspian Sea. The result was that 75% of the zooplankton was depleted, thereby affecting the entire food chain of the lake.

2006 – North and Baltic Seas

Since then, the species has apparently spread throughout the Mediterranean basin and the northwestern Atlantic. In 2006, it was first recorded in the North Sea,[6] and since October 17, 2006[7] in the western Baltic Sea, namely the Kiel Fjord and The Belts. Up to 100 animals/m³ (c. 3/cubic ft) were counted in the Baltic, whereas the population density in the North Sea was at a much lower 4 animals/m³ (about 1 animal/9 cubic ft) at most.[4]

One year later, the Baltic population of M. leidyi was found to have spread east to the Gotland Basin and the Bay of Puck.[8] The impact of the species on the already heavily stressed Baltic ecosystem is unknown. The species overwinters in the deep waters where temperature does not drop below 4 °C (39 °F); the fact that the Baltic is heavily stratified, with the waters above and below the halocline mixing little, is believed to aid its survival.[4]

Apart from the widespread P. pileus, 3 comb jelly species are occasionally drifted into the Baltic from the North Sea but do not seem to be present as a stable population of significant size: Bolinopsis infundibulum, Beroe cucumis and Beroe gracilis. The second species might potentially be used for biological control.[9]

Oliveira, Otto M. P. (2007): The presence of the ctenophore Mnemiopsis leidyi in the Oslofjorden and considerations on the initial invasion pathways to the North and Baltic Seas. Aquatic Invasions2 (3): 185–189. PDF fulltext